Ceramic-ceramic matrix composite materials are particularly useful as candle filters or baghouse filters. Candle filters are shaped like long tubes, with one open end. Such filters are fastened within an enclosure which is divided into clean and dirty sides such that the fluid to be filtered traverses from the dirty to the clean side by passing through the filter. The fluid flows typically from the outside to the inside of the filter, thus providing dust and particulate-free fluid exiting from the open end thereof.
In gas filtration, much effort has been directed to making filters capable of withstanding high temperatures and chemically corrosive environments. Ceramic materials have been found to be good candidates for filters under such conditions. However, the primary problem with ceramics is their brittleness. Hence, much of the prior art is directed to the manufacture of composite ceramic materials which, because of crack deflection and an increased flexibility, can avoid some of the traditional brittleness problems.
Many patents describe the use of ceramic materials in the filtration of particulates from hot gases. Such filters are necessary to aid in the elimination of atmospheric pollution, the recovery of desirable particulates produced in high temperature processes, such as precious metals, and the enablement of energy savings through the recycling and reuse of hot purified gas in industrial processing.
U.S. Pat. No. 4,092,194 discloses a reinforced ceramic fiber tube taught to be used as a catalyst carrier. The tube is comprised of layers of continuous ceramic fiber over which is deposited a non-porous binder comprised of an aqueous slurry of a refractory oxide precursor.
U.S. Pat. No. 4,181,514 discloses a stable filter structure for use in high temperature applications comprised of a stitch knitted high temperature fiber such as glass, ceramic or metallic yarn.
U.S. Pat. No. 4,687,697 discloses a filter of improved structural integrity wherein high temperature-resistant inorganic fibers are interlocked together to form a paper, with an inorganic fiber fabric then disposed on the paper. An adhesive is taught to hold the fabric to the paper during the formation of a pleated structure and fitting thereof into a filter frame; the adhesives and any binders that may be present are taught to burn off during use in the high temperature filtration process.
U.S. Pat. No. 4,889,630 discloses a double-layered filter wherein one layer has a coarse porosity and a second layer has a fine-grained porosity. The coarse layer is taught to be produced by forming coarse ceramic particles into a molded body followed by firing; the fine-grained layer is formed from very fine particles of ceramic materials optionally mixed with fine diameter short fibers. Materials taught to be useful for the formation of both layers include quartz, alumino-silicate, glass, aluminum oxide, silicon carbide, graphite or activated carbon, and metals.
U.S. Pat. No. 4,894,070 describes a tubular porous ceramic filter, taught to be useful for the filtration of particles from hot gases. The filter contains ceramic fibers, such as alumina, aluminosilicate, and mixtures thereof, together with a binder or colloidal oxide hydrosol. The tubes are formed by vacuum filtration of an aqueous slurry of the ceramic fibers through a wire mesh.
U.S. Pat. No. 4,917,941 discloses a ceramic composite comprised of a layer of continuous ceramic filaments surrounded by a mixture of chopped fibers, whiskers and/or particulates, which is infiltrated with a ceramic to form a matrix phase. The matrix forming material, or infiltrant, is taught to be a "meltable ceramic" (with a melting point range from about 1400.degree. C. to about 2000.degree. C.), which is comprised of alkaline earth silicates or alkaline earth alumino-silicates.
I have discovered that by manufacturing a filter from two separate parts, the filtering surface and an underlying support, the support, made from continuous ceramic fibers, provides strength and toughness to the filter construction. The filtering surface can then be prepared by winding ceramic fibers in a sufficient pattern to assure an appropriate pore size or by deposition of a ceramic fiber felt on the support surface. Silicon carbide is then coated onto the surface, typically by chemical vapor deposition.
In this way a filter having excellent strength and toughness is obtained. The filter is light in weight, a feature providing excellent economy since such filters are typically supported by steel framing.